Method for making killed steel

ABSTRACT

A method for making killed steel that permits use of high-carbon manganese without increasing the carbon content of the steel to an unacceptable level. During tapping from the furnace into the ladle, a carbon-containing manganese, and advantageously a high-carbon manganese, is first added to the molten steel, followed thereafter by addition of a strong deoxidizer. The carbon-containing manganese addition reduces the free oxygen content in the molten steel such that less of a strong deoxidizer is subsequently needed to achieve the remainder of the desired oxygen reduction.

FIELD OF THE INVENTION

[0001] This invention relates to a method for making carbon steel, and more specifically, to a method for deoxidizing molten steel in the tap ladle using high-carbon manganese prior to aluminum or silicon addition to produce killed steel.

BACKGROUND OF THE INVENTION

[0002] Steels are classified on the basis of numerous characteristics, one of which is composition. Carbon steels are one group of steels, within which the steels are loosely classified according to carbon content as low-carbon (up to 0.30% C), medium-carbon (0.30-0.60% C), or high-carbon (0.60-1.00% C). The carbon steels may further be classified as rimmed, capped, semi-killed or killed, depending on the deoxidation practice used in producing them. A rimmed steel, for example, refers to a low-carbon steel containing sufficient iron oxide to give a continuous evolution of carbon monoxide while the ingot is solidifying, resulting in a case or rim of metal virtually free of voids. Capped steel is similar to rimmed steel and is usually cast in a bottle-top ingot mold in which application of the mechanical or chemical cap renders the rimming action incomplete by causing the top metal to solidify. Semi-killed steel is incompletely deoxidized and contains sufficient dissolved oxygen, or free oxygen, to react with carbon to form carbon monoxide and thus offset solidification shrinkage. Killed steel is steel treated with a strong deoxidizing agent, such as silicon or aluminum, in order to reduce the oxygen content to such a level that no reaction occurs between carbon and oxygen during solidification.

[0003] Generally, molten steel resulting from converting a combination of molten iron and scrap steel in a basic oxygen furnace (BOF) or electric arc furnace (EAF) has to be deoxidized prior to solidification. Deoxidation occurs mainly in the steel ladle during tapping from the furnace, or thereafter at the ladle metallurgy furnace. Deoxidation is performed for the purpose of reducing the dissolved oxygen content of the molten steel to a predetermined and measurable narrow range required by the ultimate quality of the steel product. To achieve deoxidation, specified amounts of deoxidizing agents are added to the molten steel, generally, carbon, manganese, silicon and aluminum. The deoxidizing agents are generally used in combination. In addition to their deoxidizing function, the same elements may be added also for the purpose of forming an alloy with the steel to thereby alter the physical and mechanical properties thereof.

[0004] Prior to the introduction of continuous casting, ingot casting with high control of the carbon/oxygen balance was used to produce rimmed steel ingots. The carbon monoxide (CO) gas developed by the carbon/oxygen reaction during solidification was “rimmed out” leaving no porosity in the clean, virtually inclusion-free surface layers of the ingots. Rimming, however, is not readily applicable to continuous casting because the high speed of solidification overtakes the upward flow of CO bubbles, trapping large amounts of porosity in the subsurface layers of the as-cast steel. Thus, semi-killed and killed steels are generally produced by continuous casting, rather than the rimmed or capped steels. As stated above, silicon and aluminum are the main strong deoxidizing agents used in producing killed steel. Thus, killed steels are often referred to as silicon killed or aluminum killed steel. Manganese and carbon also perform a deoxidizing function, but secondary to the strong deoxidizers. In addition, the amount of free oxygen remaining in the killed steel is generally less for the aluminum killed steels than the silicon killed steels.

[0005] A typical method for producing an aluminum killed low-carbon steel may be described as follows. After the molten steel has been refined to a certain point in the EAF or BOF, it is necessary to transfer the molten steel into another vessel, called the ladle, to allow additional processing to occur. This transfer to the ladle is referred to as “tapping” the molten steel. After tapping the liquid steel to the ladle, the heat or molten steel is allowed to rim. During rimming, the carbon content is reduced by between about 0.015% to about 0.02%, depending on the oxygen levels in the heat. The percent of carbon in the molten steel before tapping from EAF or BOF to the ladle is typically between 0.035% to 0.05%. The carbon reduces during the rimming process by virtue of the oxygen searching for elements to oxidize. At this point, no alloy additions have been made to the steel, so the most abundant elements to combine with are iron and carbon. Other elements are only present in an insignificant amount. Once the carbon and oxygen combine, they form CO gas, which is then dissipated from the molten steel. The CO gas dissipation results in a reduction in the carbon content, but the extent of reduction is fairly minimal, mainly because the percent of carbon is already low when the rimming process begins in the ladle. After rimming, manganese and a strong deoxidizer, either aluminum or silicon, are added to the molten steel concurrently.

[0006] The purpose of aluminum is to deoxidize, i.e., remove oxygen from the molten steel. Deoxidation is necessary for the refinement of the steel to continue at the ladle metallurgy furnace (LMF). It is further necessary to remove the oxygen from the steel to get controlled recoveries from the alloys which are added at the EAF or BOF and at the LMF. Without the oxygen being removed, the alloys will be oxidized either to the slag or to the atmosphere, referred to above as parasitic reactions. Only when the heat is killed will the alloys revert from the slag to the steel bath. Thus, the strong deoxidizer removes the oxygen molecule from the element it was combined with, and it is effective in doing so due to a greater affinity of the strong deoxidizing elements toward oxygen than the other elements present in the heat. The heat is also killed to removed unwanted elements such as sulfur, but desulfurization is only possible when there is no oxygen present in the molten steel, and only when the slag is lime rich. Upon addition of the strong deoxidizer to the molten steel, the rimming process ends.

[0007] Despite its superior deoxidation power, aluminum deoxidation has disadvantages with respect to the products of aluminum deoxidation, which are alumina and aluminate spinel inclusions. The solid inclusions have a tendency to clog refractory nozzles and/or a significant amount of aluminum is lost to parasitic reactions with the slag. Thus, the amount of expensive aluminum added into the steel melt is based upon the minimum amount of aluminum required to deoxidize the steel plus the amount of aluminum estimated to be lost by parasitic reactions. Similar disadvantages exist with silicon deoxidation. Thus, it would be desirable to develop a method for producing a killed steel that enables less waste of the strong deoxidizing agent, such as aluminum or silicon.

[0008] As stated above, manganese and carbon are secondary oxidizers. In the steel-making process, there are three different grades of manganese, which are distinguished by their percentages of carbon. The different grades are based upon the amount of carbon pick-up when added to the steel. The carbon pick-up has been the deciding factor as to which grade of manganese will be added to make certain grades of carbon steel. Low-carbon manganese has virtually no carbon pick-up. The general industry-accepted compositions for low-carbon manganese include about 80-85 wt. % Mn and a maximum of about 0.5 wt. % C. Medium-carbon manganese generally comprises 40 pounds of carbon for every 3200 pounds of manganese, and thus has medium-carbon pick-up. The general industry-accepted compositions for medium-carbon manganese include about 80-85 wt. % Mn and about 1-1.5 wt. % C. High-carbon manganese generally comprises 40 pounds of carbon for every 570 pounds of manganese, and thus has high-carbon pick-up. The general industry-accepted compositions for high-carbon manganese include about 75-82 wt. % Mn and about 5-8 wt. % C. The percentage of carbon in the manganese directly correlates to the price for the raw material. The lower the percentage of carbon in the manganese, the more refining the raw material must undergo before being added to the molten steel. Because of the high need for refinement, low-carbon manganese is the most expensive. High-carbon manganese needs the lowest refinement, and is therefore the least expensive. As a rough estimate, low-carbon manganese typically costs twice the amount for medium-carbon manganese, and medium-carbon manganese typically costs twice the amount for high-carbon manganese. Thus, from the standpoint of cost, it would be desirable to utilize high-carbon manganese as an alloy addition and deoxidizer in the production of carbon steel.

[0009] The amount of carbon pick-up, however, in the molten steel is a limiting factor with respect to what grade of manganese is used. Low-carbon steel grades are very sensitive to the amount of high-carbon manganese due to the high-carbon pick-up. To meet the physical required characteristics of low-carbon steel grades, the carbon levels must be low. By way of example, for many low-carbon steel grades, the maximum amount of high-carbon manganese that could be used in practice with the strong deoxidizer on any 185 ton heat was 500 pounds. If more high-carbon manganese was added, the carbon levels increased to unacceptably high levels. When the carbon level increases to an unacceptably high level, the heat would have to be poured into the electric arc furnace (EAF) or basic oxygen furnace (BOF) to oxidize out the carbon and manganese. This process is expensive, creates down time in the casting line, and requires re-addition of the manganese and strong deoxidizer. It would be desirable to develop a process that would permit use of high-carbon manganese without increasing the carbon content of the steel to an unacceptably high level for the desired steel grade.

SUMMARY OF THE INVENTION

[0010] The present invention provides a method for making killed steel that permits use of high-carbon manganese without increasing the carbon content of the steel to an unacceptable level. To this end, the molten steel is tapped from the furnace into the ladle, and carbon-containing manganese, advantageously high-carbon manganese, is first added to the molten steel during tapping, followed thereafter by addition of a strong deoxidizer. By adding the carbon-containing manganese first, the carbon reacts with the free oxygen in the molten steel to produce CO gas which dissipates thereby reducing the free oxygen content in the molten steel. The remaining oxygen can then be removed by the addition of a strong deoxidizer, such as aluminum or silicon.

DETAILED DESCRIPTION

[0011] The present invention is directed to a method for producing killed carbon steel, in particular aluminum or silicon killed steel, that allows the use of high-carbon manganese and less of the strong aluminum or silicon deoxidizer than previously required, thereby lowering the cost of producing the steel without increasing the carbon content to an unacceptable level. More specifically, the present invention is directed to a method for tapping the molten steel wherein the manganese is added prior to the strong deoxidizer addition, thereby performing a two-step deoxidation. A less expensive source of manganese, i.e., a high-carbon manganese, is added to the tapped steel in a first rimming or deoxidation step during which the oxygen combines with the carbon from the high-carbon manganese as well as any carbon already in the molten steel, producing CO gas, which dissipates from the molten steel. Thus, there is concurrent reduction of carbon and oxygen in the molten bath. After the carbon and oxygen have been reduced to a desired level, the strong deoxidizer is then added to remove the remainder of free oxygen present in the molten steel. Because the oxygen content has already been partially reduced by the carbon-containing manganese addition, a lesser quantity of strong deoxidizer is needed to complete deoxidation. The more high-carbon manganese used in the first deoxidation step, the less oxygen remaining in the molten steel, and thus the less aluminum, silicon or other strong deoxidizer needed to remove the remainder of the oxygen. A reduction in the amount of strong deoxidizer used represents a significant savings. Moreover, use of high-carbon manganese rather than low-carbon manganese further represents a significant savings.

[0012] By way of definition, low-, medium-, and high-carbon manganese are terms of art wherein the composition ranges encompassed by these terms are set by the industry. It may be understood that the end points of the component ranges may vary slightly from one supplier to the next, but the following compositions are believed to be consistent with most suppliers, and therefore, generally-accepted compositions within the industry:

[0013] (a) low-carbon manganese=˜80-85 wt. % Mn, ˜0.5 wt. % maximum C;

[0014] (b) medium-carbon manganese=˜80-85 wt. % Mn, ˜1-1.5 wt. % C; and

[0015] (c) high-carbon manganese=˜75-82 wt. % Mn, ˜5-8 wt. % C.

[0016] The higher the carbon content in the manganese, the greater the effect on cost and deoxidation efficiency achieved by the present invention. Thus, the use of high-carbon manganese is preferred in the practice of the present invention. Nonetheless, use of medium- and low-carbon manganese will also achieve an improvement by virtue of the two-step process, albeit less pronounced.

[0017] By adding the high-carbon manganese before the strong deoxidizer, the oxygen in the steel is allowed to rim away not only the carbon which was in the steel at tap, but also the carbon which was added by the high-carbon manganese. This rimming occurs at the same oxygen levels and temperatures as in prior processes. However, though a large increase in the amount of carbon in the molten steel is obtained by using high-carbon manganese, the carbon levels of the final product remain the same. This is unexpectedly made possible by not adding the strong deoxidizer with the manganese. When added together, the free oxygen reacts with the aluminum first, leaving the carbon content from the manganese in the heat. In the absence of the strong deoxidizer, the rimming effect is greatly enhanced by allowing the oxygen to combine with the carbon rich manganese. While some of the manganese gets oxidized in this process, it is of little concern. The manganese oxides float to the steel surface and become part of the slag. However, once the strong deoxidizer is added, the deoxidizer removes the oxygen molecule from the manganese, thereby allowing the manganese to revert to the molten steel. The reduction of total carbon used in this two-step process can be as much as about 0.065% carbon. The previous process achieved reductions of, at most, about 0.02%. By tapping 185 ton heats using this two-step process, an increase in the amount of high-carbon manganese is achieved from, at most, 500 pounds per heat with the old, single-step process to as much as 4250 pounds per heat with the new, two-step process. For a 185 ton heat utilizing 4250 pounds of high-carbon manganese, compared to a heat using 500 pounds of high-carbon manganese, a potential savings of about $6 per ton may be achieved. For an average 185 ton tap, the total saving potential is on the order of $1,110. Further, with less oxygen in the bath after the first deoxidation step, less aluminum or other strong deoxidizer is required to remove the remainder of the oxygen. The amount of savings on aluminum depends on the amount of high-carbon manganese used, and the more high-carbon manganese, the less aluminum needed. The potential savings for aluminum may be as much as $1 a ton, or $185 for an average 185 ton tap. Thus, the total savings for an average 185 ton tap using 4250 pounds of high-carbon manganese and less aluminum strong deoxidizer is on the order of $1,300. It may be appreciated that the quantities and cost savings expressed above pertain to an exemplary 185 ton heat, but may vary for different sized heats. It may be further appreciated that the quantity of high carbon manganese to be added to a heat is dependent upon the grade of steel being produced, including the manganese specification and carbon specification. For example, a high manganese and/or high carbon specification will require a greater quantity of high carbon manganese to be added to the heat than a low manganese and/or low carbon specification.

[0018] Table 1 provides examples of oxygen reduction in four 185 ton heats, demonstrating the ability to reduce the free oxygen content to less than 5 ppm using high carbon manganese deoxidation, followed by aluminum deoxidation. Prior to tapping from the EAF to the ladle, each heat contained at least 600 ppm free oxygen, which content was reduced to below 600 ppm after high-carbon manganese addition. More specifically, for each heat, the oxygen content was reduced by 40-60% during the manganese deoxidation step. Heats 1 and 2 used 1200 pounds of high-carbon manganese, and Heats 3 and 4 used 1500 pounds. In addition, in each heat, the carbon content was less after deoxidation with the high-carbon manganese, indicating that all of the carbon present in the manganese addition is used up in the reaction with the free oxygen, as well as some of the initial carbon present before tapping. After 5-10 minutes of rimming with the manganese, aluminum was added to reduce the content of oxygen to about 1-2 ppm. TABLE 1 HEAT 1 HEAT 2 HEAT 3 HEAT 4 Oxygen Carbon Oxygen Carbon Oxygen Carbon Oxygen Carbon (ppm) (%) (ppm) (%) (ppm) (%) (ppm) (%) In EAF 758 0.044 1011 0.035 661 0.045 1069 0.036 After Mn 451 0.033  525 0.031 337 0.040  431 0.029 Addition After Al 1-2 — 1-2 — 1-2 — 1-2 — Addition

[0019] The present invention provides the greatest cost-savings potential with respect to low-carbon steel grades due to the high sensitivity to carbon pick-up. The invention is also applicable, however, to medium-carbon and high-carbon grade steels. These grades are less sensitive to carbon pick-up, and thus, have historically been able to tolerate higher amounts of medium or high-carbon manganese in the prior one-step deoxidation process. Nonetheless, by first adding high-carbon manganese, then adding the aluminum or silicon strong deoxidizer, a lesser quantity of the strong deoxidizer is needed. Thus, while a less noticeable cost savings may be experienced for the manganese addition, significant savings for the strong deoxidizer addition is still achieved.

[0020] In practice, the rim time for reducing the carbon in the first deoxidizing step is preferably at least about 5 minutes from the last manganese addition to the molten steel. The majority of oxidation of carbon occurs within the first five minutes after the manganese addition, and the reaction generally reaches equilibrium after about 10 minutes. It may be appreciated, however, that the ideal rim time is dependent upon the amount of carbon-containing manganese added and the amount of free oxygen at tap. Also in practice, the high-carbon manganese may be added in batches to avoid any foaming problems. For example, on grades which require more than 3000 pounds of manganese, foaming may occur during the rimming process if the manganese is added in a single batch. By adding the manganese in two separate batches, for example, with 5 minutes of rim time in between batches, for example, the foaming problem is eliminated. Thus, in this example, a first batch of high-carbon manganese is added, then 5 minutes elapses. Then a second batch of high-carbon manganese is added, then at least 5 minutes elapses. Then the strong deoxidizer is added. Advantageously, at least about 3 minutes is allowed to elapse between batches, and about 5-10 minutes is allowed to elapse after the final manganese addition.

[0021] While the present invention has been illustrated by the description of an embodiment thereof, and while the embodiment has been described in considerable detail, it is not intended to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus and method and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the scope or spirit of Applicant's general inventive concept. 

What is claimed is:
 1. A method for making killed steel comprising the steps of: tapping molten steel from a furnace into a ladle, wherein the molten steel comprises free oxygen; adding carbon-containing manganese to the molten steel in the ladle during tapping, whereby the carbon reacts with the free oxygen to produce CO gas which dissipates thereby reducing the free oxygen content in the molten steel; and thereafter, adding a strong deoxidizer to the molten steel in the ladle to further reduce the oxygen content in the molten steel to within a predetermined range.
 2. The method of claim 1 wherein the carbon-containing manganese is a high-carbon manganese comprising about 75-82 wt. % manganese and about 5-8 wt. % carbon.
 3. The method of claim 1, wherein the strong deoxidizer is aluminum or silicon.
 4. The method of claim 4, wherein the carbon-containing manganese is added in two batches, with at least about 3 minutes between batches.
 5. The method of claim 1, wherein adding the strong deoxidizer occurs at least about 5 minutes after adding the carbon-containing manganese.
 6. The method of claim 1, wherein adding the strong deoxidizer occurs at least about 10 minutes after adding the carbon-containing manganese.
 7. The method of claim 1, wherein the molten steel comprises at least about 600 ppm free oxygen prior to adding the carbon-containing manganese and less than about 600 ppm after adding the carbon-containing manganese.
 8. The method of claim 1, wherein the free oxygen content is reduced by at least about 40% after adding the carbon-containing manganese.
 9. The method of claim 1, wherein the predetermined range is 0 to about 5 ppm.
 10. A method for making Al/Mn killed steel comprising the steps of: tapping molten steel from a furnace into a ladle, wherein the molten steel comprises free oxygen in an amount of at least about 600 ppm; adding a high-carbon manganese to the molten steel during tapping whereby the carbon reacts with the oxygen for a period of at least about 5 minutes to produce CO gas which dissipates, thereby reducing the oxygen content in the molten steel to less than about 600 ppm; thereafter, adding an aluminum-containing deoxidizer to further reduce the oxygen content in the molten steel to about 5 ppm or less.
 11. The method of claim 10, wherein the high-carbon manganese is added in an amount of at least about 3000 pounds per 185 tons of the molten steel.
 12. The method of claim 11, wherein the high-carbon manganese is added in two batches, with at least about 3 minutes between batches.
 13. The method of claim 10, wherein the free oxygen content is reduced by at least about 40% after adding the high-carbon manganese.
 14. The method of claim 10 wherein the carbon reacts with the oxygen for a period of about 5-10 minutes. 